1. Field of the Invention
The present invention relates to apparatus for the measurement of straightness of travel. More particularly, the invention relates to optical apparatus which is useful for high accuracy straightness of travel metrology using interferometry.
2. The Prior Art
High accuracy straightness of travel measurements are increasingly important in the machine tool industry to determine the precise geometry of machine slides. Stringent requirements also exist in the semiconductor fabrication industry to determine the geometry of precision X-Y stages. The short straight edge and the optical flat have been two prior art tools used for measuring the geometry of short axes. Also, a straightness of travel interferometer described in R. R. Baldwin, B. E. Grote, and D. A. Harland, "A Laser Interferometer that Measures Straightness of Travel," Hewlett-Packard Journal, Vol. 25, pp. 10-20, January 1974, has been used for the measurement of longer axes.
The prior art straightness of travel interferometer suffers from a number of limitations, in particular: (1) the input and return laser beams are coincident so that it is possible to destabilize the frequency and output power of the laser source, (2) it uses a conventional 50/50 beamsplitter so that it has poor light efficiency; (3) it requires an expensive Wollaston prism, (4) it requires an expensive target mirror assembly whose angle must be matched precisely to the Wollaston prism, and (5) it is very difficult to use since the angular orientation of the target mirror assembly must be maintained very precisely to optical tolerances during the measurements.
In the present invention, (1) the input and return laser beams are offset, (2) it has high light efficiency, (3) no Wollaston prisms are required, (4) it is not necessary to match the angles of the target mirror assembly to another element precisely, and (5) it is easy to use since it does not require critical initial alignment and is insensitive to angular variations during travel. The improvements of the present invention thusly overcome the disadvantages of the prior art and allow the high accuracy, i.e., to a fraction of a micrometer per meter, straightness of travel measurement required for machine tool slides and precision X-Y stages.